This invention is primarily applicable to the drive trains of vehicles wherein drive power is provided for both front and rear wheels. More particularly, it is applicable to vehicles that have one set of wheels permanently engaged and the other set of wheels engaged as needed, in either direction of travel.
A vehicle may have the rear wheels permanently engaged and under favorable driving conditions the front wheels are not engaged. Should the rear wheels start to slip, e.g., when driving on ice or snow or when trying to get unstuck from a stuck condition, the front wheels automatically engage.
Such engagement is provided by a clutch mechanism interposed between a component of the vehicles drive train connected to the permanently driven wheels (drive component) and a component of the drive train connected to the normally passive wheels (driven component). When slipping starts to occur, the rear wheels rotate faster than the front (passive) wheels and this translates into the drive component rotating faster than the driven component.
The clutch mechanism of the invention reacts to such overrunning rotation of the drive component and causes the clutch mechanism to inter-engage the drive and driven components thus imparting drive power to the normally passive wheels.
A known clutch mechanism designed for this purpose is disclosed in U.S. Pat. No. 5,195,604. Rollers retained in a cage are interposed between a drive ring and a driven ring mounted in concentric relation. The annular spacing between the two rings (in which the rollers reside) is configured so that the radial distance between the rings varies between positions where the spacing between the ring is greater than the diameter of the rollers and where the distance is smaller than the diameter of the rollers. When overrunning occurs, the rollers move to the positions of smaller spacing and become wedged between the rings and thereby interlock the two rings. This occurs in either forward or reverse direction.
Roller clutches have some disadvantages, however. The wedgetype of interlocking generates radial stresses that require relatively massive drive and driven rings. This adds weight and takes up room in areas where such characteristics are undesirable. The present invention is directed to the replacement of the roller clutch mechanism with a pawl clutch mechanism. The use of a pawl clutch mechanism for the purposes described above is disclosed in U.S. Pat. No. 4,222,473. The pawl clutch mechanism of the '473 patent does not automatically (inherently) shift between front and reverse drive and requires the positive shifting (manual activation) of a phasing ring, e.g., via a solenoid. An even earlier patent (U.S. Pat. No. 2,181,244) discloses a pawl clutch mechanism as applied to locomotives. A complex and massive mechanism includes forward and rearward directed pawls mounted on trunnions and connected to forward and rearward drag members via trunnion arms. The pawls are pivoted by the resistive action of drag members, one set of pawls is pivoted into engagement and another set out of engagement depending on the direction of rotation.
It is an objective of the present invention to provide inherent shifting of a pawl type clutch mechanism in either direction of rotation produced by a more simple and efficient design (as compared to the prior mechanisms) and to provide this design in a compact package as required for vehicle application.